1. Field of the Invention
The present invention relates to a magnetic disk drive apparatus, an optical disk drive apparatus, a magnetic tape drive apparatus, or the like apparatus and more particularly, to an active filter control apparatus which performs optimum waveform equalization of a read-out waveform in a signal processor of a system having different data transfer rates, e.g., at the inner and outer peripheries of a disk.
2. Description of the Related Art
A constant density recording (hereinafter simply referred to as CDR) method has been devised as a method for increasing the recording capacity of a magnetic disk. When the CDR method is adopted, the rate of transferring data between the disk and a signal processing unit changes, and therefore an active filter whose filter characteristic is variable with the transfer rate is employed as a filter for processing a read-out waveform. Sympathetically with the tendency towards increasing the capacity of the magnetic disk, the data transfer rate tends to increase and a highly accurate active filter operable to a high frequency band is sought. Further, with a system using the a magnetic disk drive apparatus, such as a personal computer, a reduction in operation voltage and power necessitates reduction in operation voltage and power consumption of the magnetic disk drive apparatus, and so the active filter is also required to perform with reduced in operation voltage and power consumption.
A conventional active filter control apparatus will be described with reference to FIG. 1.
FIG. 1 is a block diagram schematically showing the conventional active filter control apparatus. The apparatus comprises a microprocessor 1 and an active filter block 100 which includes a register 2, a digital to analog converter (hereinafter referred to as DAC) 5, a transconductance control current generator (hereinafter simply referred to as Gm control current generator) 8, an active filter 9 and a reference current source 10. The active filter 9 includes transconductance amplifiers (hereinafter simply referred to as Gm amplifiers) 11 and 12 and capacitors 13 and 14.
When the active filter control apparatus is used in the magnetic disk drive apparatus, the microprocessor 1 sets a value in the register 2 in accordance with a data transfer rate, and the DAC 5 receives a reference current iref from the reference current source 10 and delivers a cut-off frequency control current ifc complying with the set value of the register 2. The Gm control current generator 8 receives the cut-off frequency control current ifc and delivers transconductance control currents igm1 and igm2 which are proportional to the input control current ifc. The ratio between the transconductance control currents igm1 and igm2 is determined by a characteristic of the active filter 9. Conductance of the Gm amplifiers 11 and 12 in the active filter 9 is controlled using the transconductance control currents igm1 and igm2 to control the filter characteristic of the active filter 9.
The active filter control apparatus as above is described in, for example, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL.27, NO.3, MARCH 1992, "Design of a Bipolar 10-MHz Programmable Continuous-Time 0.05.degree. Equiripple Linear Phase Filter".
The filter characteristic of the prior art active filter will be described.
FIG. 2 is a graph showing the relation between the set value of the cut-off frequency setting register and the cut-off frequency (fc) in the prior art active filter. The dotted line represents the characteristic of an ideal active filter, and the solid line represents the characteristic of a real circuit. While in a low cut-off frequency band the ideal characteristic coincides with the characteristic of the real circuit, the cut-off frequency in the real circuit decreases below that of the ideal characteristic in a high cut-off frequency band, and linearity is lost. This is because the pole existing parasitically in the Gm amplifiers constituting the active filter has less effect on the cut-off frequency of the active filter in the low frequency band than in the high frequency band.
FIG. 3 is a graph showing the relation between the cut-off frequency and Q in the active filter. The dotted line represents the ideal characteristic, and the solid line represents the characteristic of the real circuit. The Q of filter ideally remains constant even when the cut-off frequency changes, but in the real circuit, when the cut-off frequency is increased, the phase rotation is increased under the influence of the pole in the Gm amplifiers and Q becomes large.